Polybenzimidazole (pbi) membranes for redox flow batteries

ABSTRACT

Disclosed are redox flow battery membranes, redox flow batteries incorporating the membranes, and methods of forming the membranes. The membranes include a polybenzimidazole gel membrane that is capable of incorporating a high liquid content without loss of structure that is formed according to a process that includes in situ hydrolysis of a polyphosphoric acid solvent. The membranes are imbibed with a redox flow battery supporting electrolyte such as sulfuric acid and can operate at very high ionic conductivities of about 100 mS/cm or greater. Redox flow batteries incorporating the PBI-based membranes can operate at high current densities of about 100 mA/cm2 or greater.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 16/570,290, having a filing date of Sep. 13, 2019, which claimsfiling benefit of U.S. Provisional Patent Application Ser. No.62/731,156, titled “Polybenzimidazole (PBI) Gel Membranes for FlowBatteries,” having a filing date of Sep. 14, 2018, all of which areincorporated herein by reference for all purposes.

FEDERAL RESEARCH STATEMENT

This invention was made with Government support under Grant No.DE-AR0000767, awarded by the Department of Energy. The Government hascertain rights in the invention.

BACKGROUND

Increasing demands on the energy sector have created a new need forlarge-scale energy storage devices with additional implications in gridmanagement and back-up power coincidentally with the seamlessintegration of new renewable energy devices. Redox flow batteries havean as yet unmet potential to efficiently store large amounts of energyas well as meet cost requirements for meeting such needs. Redox flowbatteries perform charging and discharging by using a positive electrodeelectrolyte solution and a negative electrode electrolyte solutionseparated by an ion exchange membrane, each electrolyte solutioncontaining metal or organic ions (active materials) that form a redoxpair (alternatively referred to as a redox couple) by which valence ischanged by oxidation-reduction.

Unfortunately, the widespread adoption of redox flow batteries has beenlimited by the high cost of device fabrication. For instance, in avanadium redox flow battery (VRB) a major portion of the cost isattributed to the vanadium electrolytes. Such costs could be mitigatedby reducing the size of the electrochemical stack. However, toaccomplish this effectively and maintain high power densities thebattery must be able to operate under high current loads. Traditionalmembranes used in redox flow batteries (generally perfluorosulfonic acidmembranes such as Nafion®) are poor ionic conductors and are unable tosustain operation under high current loads, and thus the batteriesincorporating these membranes require larger cell stacks at highercosts. In an attempt to reduce costs of redox flow batteries andincrease overall performance, there has been a surge in membranedevelopment activities, with limited results.

The ion exchange membrane is a primary component of a redox flow batteryand has an important effect upon the output, capacity, lifespan and costof the battery. In addition to a requirement for low crossover of redoxpair electrolyte ions as well as ability to function at high currentloads, the ion exchange membrane must exhibit mechanical and chemicalstability and high durability. The ion exchange membrane of a redox flowbattery is always immersed in an electrolyte solution, and thus must beable to resist degradation due to oxidation or the like, and thedurability of the membrane becomes a main factor determining thelifespan of a redox flow battery.

Certain types of phosphoric acid (PA) doped polybenzimidazole (PBI)membranes have been considered for use in a variety of electricalapplications. Traditional PBI membranes are most notably known for theirperformance as high temperature polymer electrolyte membranes. Thesetraditional PBI membranes have also been considered for multiple newdevices, such as electrochemical hydrogen separation, SO₂ depolarizedelectrolyzers, and redox flow batteries. To date, research on PBImembranes for redox flow batteries has focused on the traditionalmeta-polybenzimidazole (m-PBI) and its derivatives. Traditional PBImembranes for use in electrochemical applications have been prepared bysolution casting in N,N′-dimethylacetamide (DMAc) to form a dense filmfollowed by imbibing the formed film in the desired electrolyte, coinedthe “conventional imbibing process.” Unfortunately, these conventionalPBI membranes have been shown to exhibit extremely low ionicconductivities when imbibed in electrolyte solutions (less than 20mS·cm⁻¹) and an inability to operate at current loads above about 100 mAcm⁻². Moreover, the conventional imbibing process for traditional PBImembranes is a time consuming, environmentally unfriendly technique thatadds cost to the membrane fabrication process.

More recently, a process to prepare PBI gel membranes has been developedthat includes direct casting of a polymerization composition comprisingthe PBI polymer in polyphosphoric acid (PPA) solvent. Subsequenthydrolysis of the PPA solvent to PA, which is a poor solvent for PBI,induces solidification of the PBI gel membrane that is imbibed as-formedwith PA.

What is needed in the art is an ion exchange membrane for a redox flowbattery that exhibits high ionic conductivity and that can operate underhigh current loads while also being highly stable and durable in thechallenging environment of the redox flow battery.

SUMMARY

According to one embodiment, disclosed is a method for forming a redoxflow battery membrane. The method can include forming a polymerizationcomposition, the polymerization composition including a polyphosphoricacid (PPA), an aromatic or heteroaromatic tetraamino compound, and anaromatic or heteroaromatic carboxylic acid compound. The aromaticcarboxylic acid compound can be an aromatic or heteroaromaticpolycarboxylic acid or ester, anhydride, or acid chloride thereof or anaromatic or heteroaromatic diaminocarboxylic acid. The method alsoincludes polymerizing the compounds of the polymerization composition,e.g., via heating, to effect polymerization of the compounds andformation of a polybenzimidazole (PBI) solution.

Following polymerization, the PBI polymer solution can be shaped to forma membrane precursor and the PPA of the solution can be hydrolyzed,thereby forming phosphoric acid (PA) and water and solidifying thepolymer of the membrane precursor to form a PBI gel membrane imbibedwith PA and water. The PBI gel membrane differs from previously knowntraditional PBI membranes, as it can incorporate high liquid content andretain structure, i.e., capable of maintaining a self-supportingstructure even at a low solids content of, e.g., about 40 wt. % or less.The method can further include imbibing the PBI gel membrane with aredox flow battery supporting electrolyte, one example of which beingsulfuric acid. For example, the as-formed PBI gel membrane can be washedwith water several times to remove PA while maintaining a high liquidcontent of the as-formed PBI gel membrane (generally about 60 wt. % ormore) and can then be contacted with a solution including a redox flowbattery supporting electrolyte, thereby imbibing the PBI gel membranewith the supporting electrolyte and forming the redox flow batterymembrane.

Also disclosed is a redox flow battery membrane that includes a PBI gelmembrane (i.e., a PBI membrane that is self-supporting at high liquidcontent) and a redox flow battery supporting electrolyte imbibed in thePBI gel membrane. The redox flow battery membrane can exhibit a highionic conductivity, e.g., about 100 mS/cm or higher in a 2.6 M sulfuricacid solution.

Redox flow batteries incorporating the redox flow battery membranes arealso described. A redox flow battery can include a redox flow batterymembrane as described separating an anolyte solution and a catholytesolution. Disclosed redox flow batteries can operate at high currentdensities, e.g., about 100 mA/cm² or greater. A redox flow battery caninclude other battery components as are known in the art, e.g.,electrodes, current collectors, flow lines, etc. and can include asingle cell or multiple cells in a single cell stack or multiple cellstacks.

BRIEF DESCRIPTION OF THE FIGURES

A full and enabling disclosure of the present subject matter, includingthe best mode thereof to one of ordinary skill in the art, is set forthmore particularly in the remainder of the specification, includingreference to the accompanying figures in which:

FIG. 1 schematically illustrates a redox flow battery as may incorporatea membrane as described herein.

FIG. 2 schematically illustrates a multi-cell stack as may be includedin a redox flow battery as described.

FIG. 3 presents the voltage efficiency (VE), the coulombic efficiency(CE) and the energy efficiency (EE) of a VRB incorporating a membrane asdescribed herein.

FIG. 4 presents the polarization curve for the VRB of FIG. 3 compared toa VRB incorporating a traditional PBI membrane.

FIG. 5 presents the VE, the CE and the EE of a VRB incorporating amembrane as described herein.

FIG. 6 presents the polarization curve for the VRB of FIG. 5 compared toa VRB incorporating a traditional PBI membrane.

FIG. 7 presents the VE, the CE and the EE of a VRB incorporating amembrane as described herein.

FIG. 8 presents the VE, the CE and the EE of a VRB incorporating amembrane as described herein.

FIG. 9 presents the VE, the CE and the EE of a VRB incorporating amembrane as described herein.

FIG. 10 presents the VE, the CE and the EE of a VRB incorporating acomparison membrane.

FIG. 11 compares the polarization curves for a VRB incorporating acomparison membrane with two other batteries, each incorporating amembrane as described herein.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thedisclosed subject matter, one or more examples of which are set forthbelow. Each embodiment is provided by way of explanation of the subjectmatter, not limitation thereof. In fact, it will be apparent to thoseskilled in the art that various modifications and variations may be madein the present disclosure without departing from the scope or spirit ofthe subject matter. For instance, features illustrated or described aspart of one embodiment, may be used in another embodiment to yield astill further embodiment.

The present disclosure is generally directed to redox flow battery ionexchange membranes, methods for forming the membranes, and redox flowbatteries incorporating the membranes. The redox flow battery membranesare based upon polybenzimidazole (PBI) membranes, and more specifically,PBI gel membranes. As utilized herein, the term “gel” generally refersto a polymeric matrix that can incorporate a high liquid content andmaintain a self-supporting structure. For instance, a PBI gel membraneas described herein can incorporate about 60 wt. % or more, about 65 wt.% or more, about 75 wt. % or more, about 80 wt. % or more, or about 85wt. % or more liquid by weight of the composite membrane (total solidsplus liquid content) without loss of structure of the polymeric matrix.In one embodiment, a PBI gel membrane can incorporate from about 60 wt.% to about 95 wt. % liquid while maintaining a self-supporting,semi-rigid structure, i.e., capable of being manipulated and exhibitingpliability without loss of structure of the polymeric matrix. Inaddition, a PBI gel membrane can be processed to remove liquid from thegel and then re-imbibed with a liquid to re-swell without loss ofstructure of the polymeric matrix.

The redox flow battery membranes can exhibit high conductivity and lowcell resistance, allowing operation under high current load conditionswith high performance, which can translate to batteries with smaller andless costly electrochemical stacks providing the same or betterperformance as compared to other technologies. For instance, a redoxflow battery membrane as described can exhibit an in-plane ionicconductivity in a 2.6 M sulfuric acid solution of about 100 mS/cm orgreater, about 200 mS/cm or greater, or about 300 mS/cm or greater insome embodiments. Crosslinked membranes can exhibit extremely highin-plane ionic conductivity, such as about 300 mS/cm or greater, about400 mS/cm or greater, or about 500 mS/cm or greater. For instance, aredox flow battery can exhibit an in-plane ionic conductivity in a 2.6 Msulfuric acid solution of from about 100 mS/cm to about 600 mS/cm insome embodiments.

In addition, batteries incorporating disclosed membranes can operate athigh current density, for instance, about 100 mA/cm² or higher, e.g.,from about 100 mA/cm² to about 500 mA/cm² in some embodiments. Moreover,batteries incorporating a redox flow battery membrane as described canoperate at high efficiency. By way of example, at a current density of242 mA/cm² a redox flow battery incorporating a membrane as describedcan exhibit a coulombic efficiency (CE) of about 90% or greater, forinstance, from about 93% to about 99% in some embodiments; an energyefficiency (EE) of about 75% or more, for instance, from about 78% toabout 85% in some embodiments; and a voltage efficiency (VE) of about80% or more, for instance, from about 81% to about 87%. At a currentdensity of 483 mA/cm², a redox flow battery as described can exhibit aCE of 90% or greater, for instance, from about 94% to about 98% in someembodiments; an EE of about 65% or more, for instance, from about 65% toabout 75% in some embodiments; and a VE of about 65% or more, forinstance, from about 66% to about 77%.

The performance characteristics of the redox flow battery membranes arebased on the use of PBI gel membranes. Batteries and battery cells thatincorporate the membranes are much improved as compared to thoseincorporating ion exchange membranes based upon conventional PBI polymermembranes, which exhibit very low ionic conductivities, e.g., less than20 mS/cm. Moreover, batteries that incorporate conventional PBI polymermembranes are unable to function at current densities above about 80mA/cm². Without wishing to be bound to any particular theory, it isbelieved that the excellent characteristics of the disclosed redox flowbattery membranes are due to the morphology of the polymeric matrix thatforms the membrane structure. As described further herein, the disclosedgel membranes are formed according to a method that includes hydrolysisof the PPA polymer solvent and the subsequent solidification of the PBIpolymer in the hydrolysis product (PA). It is believed that this in situhydrolysis and polymer solidification leads to formation of an orderedpolymeric matrix that differs in molecular structure from thetraditional, organic solution-cast PBI membranes that are cast as asolution of an organic solvent followed by solidification by removal ofthe organic solvent via, e.g., heating. In particular, it is believedthat the PBI gel membrane structure includes a more open and orderedframework as compared to traditional PBI membranes, with the frameworkof the PBI matrix providing a stable gel membrane that exhibits theimproved electrochemical properties as described.

In addition to highly desirable electrochemical characteristics, theredox flow battery membranes based on PBI polymers are highly resistantto degradation in the redox flow battery environment. For instance,disclosed membranes can show little or no degradation in oxidativevanadium solutions. As such, disclosed membranes can provide long-termactivity, further reducing costs of redox flow batteries thatincorporate the membranes.

To form a PBI gel membrane for use as a redox flow battery membrane, apolymerization composition can be formed that includes a PPA and thePBI-forming compounds of choice, e.g., PBI-forming monomers. The monomercontent of the polymerization composition can generally be low, forinstance, about 10 wt. % or less, about 8 wt. % or less, or about 5 wt.% or less in some embodiments.

The PBI polymer of the PBI gel membrane can have any PBI structure as isgenerally known in the art and can be formed by polymerization ofPBI-forming compounds including at least one aromatic or heteroaromatictetraamino compound and at least one aromatic or heteroaromaticpolycarboxylic acid or ester, anhydride, or acid chloride thereof or atleast one aromatic or heteroaromatic diaminocarboxylic acid.Heteroaromatic compounds encompassed herein include aromatic systemsthat contain at least one nitrogen, oxygen, sulfur or phosphorus atom inan aromatic ring.

Examples of aromatic and heteroaromatic tetraamino compounds as may beutilized in forming the PBI gel membrane can include, withoutlimitation, 2,3,5,6-tetraminopyridine;3,3′,4,4′-tetraminodiphenylsulfone; 3,3′,4,4′-tetraminodiphenyl ether;3,3′,4,4′-tetraminobiphenyl; 1,2,4,5-tetraminobenzene;3,3′,4,4′-tetraminobenzophenone; 3,3′,4,4′-tetraminodiphenylmethane; and3,3′,4,4′-tetraminodiphenyldimethyl-methane and the salts thereof, e.g.,the mono-, di-, tri- and tetrahydrochloride salts, as well as anycombination of aromatic or heteroaromatic tetraamino monomers.

In one embodiment, an aromatic polycarboxylic acid can include adicarboxylic acid. A dicarboxylic acid can be utilized alone or incombination with one or more additional polycarboxylic acid compounds,e.g., tricarboxylic acids and/or tetracarboxylic acids. Whenincorporated, the content of tricarboxylic acid or tetracarboxylic acidscan generally be about 30 mol % or less, for instance, from about 0.1mol % to about 20 mol %, or from about 0.5 mol % to about 10 mol % basedon the amount of one or more dicarboxylic acid compounds. An ester of apolycarboxylic acid can be utilized such as C1-C20-alkyl esters orC5-C12-aryl esters of a polycarboxylic acid. An anhydride of apolycarboxylic acid or an acid chloride of a polycarboxylic acid can bepolymerized according to disclosed methods.

Examples of aromatic dicarboxylic acids can include, without limitation,pyridine-2,5-dicarboxylic acid; pyridine-3,5-dicarboxylic acid;pyridine-2,6-dicarboxylic acid; pyridine-2,4-dicarboxylic acid;4-phenyl-2,5-pyridinedicarboxylic acid; 3,5-pyrazoledicarboxylic acid;2,6-pyrimidinedicarboxylic acid; 2,5-pyrazinedicarboxylic acid;2,4,6-pyridinetricarboxylic acid; benzimidazole-5,6-dicarboxylic acid;5-hydroxyisophthalic acid; 4-hydroxyisophthalic acid;2-hydroxyterephthalic acid; 5-aminoisophthalic acid;5-N,N-dimethylaminoisophthalic acid; 5-N,N-diethylaminoisophthalic acid;2,5-dihydroxyterephthalic acid; 2,6-dihydroxyisophthalic acid;4,6-dihydroxyisophthalic acid; 2,3-dihydroxyphthalic acid;2,4-dihydroxyphthalic acid; 3,4-dihydroxyphthalic acid;1,8-dihydroxynaphthalene-3,6-dicarboxylic acid;diphenylsulfone-4,4′-dicarboxylic acid; isophthalic acid; terephthalicacid; phthalic acid; 3-fluorophthalic acid; 5-fluoroisophthalic acid;2-fluoroterephthalic acid, tetrafluorophthalic acid,tetrafluoroisophthalic acid, tetrafluoroterephthalic acid;3-sulfophthalic acid; 5-sulfoisophthalic acid; 2-sulfoterephthalic acid;tetrasulfophthalic acid; tetrasulfoisophthalic acid;tetrasulfoterephthalic acid; 1,4-naphthalenedicarboxylic acid;1,5-naphthalenedicarboxylic acid; 2,6-naphthalenedicarboxylic acid;2,7-naphthalenedicarboxylic acid; diphenic acid; diphenyl ether4,4′-dicarboxylic acid; benzophenone-4,4′-dicarboxylic acid;biphenyl-4,4′-dicarboxylic acid; 4-trifluoromethylphthalic acid;2,2-bis(4-carboxyphenyl)hexafluoropropane; 4,4′-stilbenedicarboxylicacid; and 4-carboxycinnamic acid or any combination thereof.

Examples of aromatic tricarboxylic acids and esters, acid anhydrides,and acid chlorides thereof can include, without limitation,1,3,5-benzenetricarboxylic acid (trimesic acid);1,2,4-benzenetricarboxylic acid (trimellitic acid);(2-carboxyphenyl)iminodiacetic acid; 3,5,3′-biphenyltricarboxylic acid;and 3,5,4′-biphenyltricarboxylic acid; or any combination thereof.

Examples of aromatic tetracarboxylic acids and esters, acid anhydrides,and acid chlorides thereof can include, without limitation,3,5,3′,5′-biphenyltetracarboxylic acid; benzene-1,2,4,5-tetracarboxylicacid; benzophenonetetracarboxylic acid;3,3′,4,4′-biphenyltetracarboxylic acid;2,2′,3,3′-biphenyltetracarboxylic acid;1,2,5,6-naphthalenetetracarboxylic acid; and1,4,5,8-naphthalenetetracarboxylic acid; or any combination thereof.

Heteroaromatic carboxylic acids can include heteroaromatic dicarboxylicacids, heteroaromatic tricarboxylic acids and heteroaromatictetracarboxylic acids, including their respective esters such asC1-C20-alkyl esters, C5-C12-aryl esters, or the acid anhydrides or theacid chlorides of the heteroaromatic carboxylic acids. Examples ofheteroaromatic carboxylic acids include, without limitation,pyridine-2,5-dicarboxylic acid; pyridine-3,5-dicarboxylic acid;pyridine-2,6-dicarboxylic acid; pyridine-2,4-dicarboxylic acid;4-phenyl-2,5-pyridinedicarboxylic acid; 3,5-pyrazoledicarboxylic acid;2,6-pyrimidinedicarboxylic acid; 2,5-pyrazinedicarboxylic acid;2,4,6-pyridinetricarboxylic acid; benzimidazole-5,6-dicarboxylic acid;and also their C1-C20-alkyl esters or their C5-C12-aryl esters; or theiracid anhydrides or their acid chlorides; or any combination thereof.

In one embodiment, the polymerization composition can include adiaminocarboxylic acid, examples of which include, without limitation,diaminobenzoic acid and the mono and dihydrochloride derivatives of saidacid, as well as 1,2-diamino-3′-carboxy acid 4,4′-diphenyl ether, or anycombination thereof.

PPA as can be utilized in the polymerization composition can becommercial PPA as obtainable, for example, from Riedel-de Haen. PPA caninclude concentrated grades of PA (H₃PO₄) above 100%. At highconcentrations, the individual H₃PO₄ units are polymerized bydehydration and the PPA can be expressed by the formulaH_(n+2)P_(n)O_(3n+1) (n>1).

The PPA [H_(n+2)P_(n)O_(3n+1) (n>1)] can have a P₂O₅ content ascalculated by acidimetry of about 70 wt. % or more, for instance, about75 wt. % or more, or about 82 wt. % or more, for instance, from about 70wt. % to about 86 wt. % in some embodiments. The polymerizationcomposition can be in the form of a solution of the monomers/compounds,or a dispersion/suspension of the monomers/compounds in the PPA,generally depending upon the nature of the compounds to be polymerizedand any additional components of the polymerization solution.

The polymerization can be carried out at a temperature and for a timeuntil suitable polymerization of the compounds has taken place, whichcan generally be determined by an increase in viscosity of thepolymerization composition. The increase in viscosity can be determinedby visual inspection, through determination of the intrinsic viscosity,or by any other suitable means. For instance, the polymerization cancontinue until the polymerization composition exhibits an intrinsicviscosity of about 0.8 dL/g or greater, for instance, about 1.0 dL/g orgreater, or about 1.5 dL/g or greater, in some embodiments. Thepolymerization temperature can generally be about 220° C. or less, forinstance, about 200° C. or less, such as about 100° C. to 195° C. insome embodiments. The polymerization can be carried out over a time offrom a few minutes (e.g., about 5 minutes) up to several hours (e.g.,about 100 hours). In one embodiment, the polymerization composition canbe heated in a stepwise fashion, for instance, in three or more steps,each step lasting from about 10 minutes to about 5 hours and increasingthe temperature by about 15° C. or more for each step. Of course, theparticular polymerization conditions can be varied, depending generallyupon the reactivity and concentration of the particular monomers, aswould be evident to one of skill in the art, and no particularpolymerization conditions are required in formation of the redox flowbattery membranes.

Exemplary PBI polymer repeating units of a PBI gel membrane can include,without limitation:

or any combination thereof, in which n and m are each independently 1 orgreater, about 10 or greater, or about 100 or greater, in someembodiments.

A PBI polymer of a membrane as disclosed herein can include anyrepeating unit including any derivatization thereof as is generallyknown in the art, examples of which are well within the knowledge of oneof skill in the art, representative examples of which have beendescribed, for instance, in US Patent Application Publication No.2013/0183603 to Benicewicz, et al., which is incorporated by referenceherein.

Following polymerization, the polymer can be in solution in the PPAsolvent, and the PBI polymer solution can be processed to form a gelmembrane precursor having a desired thickness. Beneficially, the polymersolution, as well as the gel membrane precursor and eventual gelmembrane and redox flow battery membrane formed of the polymer solution,can be free of organic solvents.

The membrane precursor can be formed according to any suitable formationprocess, such as, and without limitation to, casting, spray coating,knife coating, etc. For instance, the gel membrane precursor can beformed to a thickness of from about 20 micrometers (μm) to about 4,000μm in one embodiment, such as from about 30 μm to about 3,500 μm, orfrom about 50 μm to about 1,000 μm, in some embodiments.

To solidify the polymer and form the PBI gel membrane, the PBI polymersolution can be treated in the presence of water and/or moisture tohydrolyze at least a portion of the PPA of the solution. Uponhydrolysis, the PPA will hydrolyze to form PA and water, thereby causinga sol-gel transfer of the PBI polymer solution and solidification of thepolymer, as the PBI polymer is less soluble in PA as compared to PPA.

The hydrolysis treatment can be carried out at temperatures and for atime sufficient for the gel membrane to solidify so as to beself-supporting and capable of being manipulated without destructionwhile incorporating high liquid content (e.g., about 60 wt. % or higherliquid content of the total solid and liquid content of the membrane).By way of example, the hydrolysis treatment can be carried out at atemperature of from about 0° C. to about 150° C., for instance, fromabout 10° C. to about 120° C., or from about 20° C. to about 90° C.,e.g., at ambient temperature in some embodiments (e.g., at a relativehumidity contacting environment of from about 35% to 100%).

The hydrolysis can be carried out by contact of the gel membraneprecursor with H₂O, for instance, in the form of a liquid or vapor,and/or in the presence of other components. For instance, the gelmembrane precursor can be contacted with water vapor and/or liquid waterand/or steam and/or aqueous PA (e.g., a PA solution having a PAconcentration of from about 10 wt. % to about 90 wt. %, e.g., about 30wt. % to about 70 wt. % or about 45 wt. % to about 55 wt. %). Thetreatment can be carried out under standard pressure, but this is not arequirement of a formation process, and in some embodiments, thehydrolysis treatment can be carried out under a modified pressure.

In one embodiment, the hydrolysis can be carried out in aclimate-controlled environment in which the H₂O content can be tightlycontrolled. For example, the moisture content of the local environmentcan be controlled through control of the temperature or saturation ofthe fluid contacting the precursor membrane. For example, carrier gasessuch as air, nitrogen, carbon dioxide or other suitable gases can carryH₂O, e.g., steam, in a controlled amount for contact with the precursormembrane.

The hydrolysis treatment time can generally vary depending uponparameters such as, e.g., H₂O content and form of the contact, membranethickness, contact temperature, etc. In general, the hydrolysistreatment can be carried out in a time period of between a few secondsto a few minutes, for instance, when the hydrolysis treatment utilizessuperheated steam, or alternatively over a period of several days, forexample when the hydrolysis treatment is carried out at ambienttemperature and low relative atmospheric moisture. In some embodiments,the hydrolysis treatment can be carried out over a period of timebetween about 10 seconds and about 300 hours, for instance, from about 1minute to about 200 hours. By way of example, in an embodiment in whichthe at least partial hydrolysis of the PPA of the PBI polymer solutionis carried out at room temperature (e.g., about 20° C.) with ambient airof relative atmospheric moisture (i.e., relative humidity) content offrom about 20% to 100%, for instance, from about 40% to about 80%, thetreatment time can generally be between about 5 hours and about 200hours.

Upon hydrolysis of at least a portion of the PPA of the PBI polymersolution, the polymer can solidify, which form the PBI gel membrane. ThePBI gel membrane can, in one embodiment, have a thickness of from about15 μm to about 3000 μm, for instance, from about 20 μm to about 2000 μm,or from about 20 μm to about 1500 μm, though any particular membranethickness is not critical. In some embodiments, the PBI gel membrane canhave a thickness that is less than that of the membrane precursor. Aspreviously discussed, following hydrolysis, the PBI gel membrane can beself-supporting, even at high liquid content, which is believed to bedue to the intra- and intermolecular polymer structures present in thesolidified polymeric matrix.

The as-formed PBI gel membrane can in one embodiment have PBI solidscontent of from about 5 wt. % to about 40 wt. %, for instance, fromabout 8 wt. % to about 30 wt. %, or from about 10 wt. % to about 25 wt.% of the total weight of the membrane including liquid content. Theas-formed PBI gel membrane can be self-supporting, for instance, havinga Young's modulus of about 2.0 MPa or greater, for instance, about 3.0MPa or greater, or about 4.5 MPa or greater in some embodiments asdetermined for a PBI gel membrane having a thickness of 0.43 mm and aPBI content of 5 wt. % (e.g., polybenzimidazole).

To form the redox flow battery membrane from the PBI gel membrane, thePA and any remaining PPA incorporated in the membrane can be removed andreplaced with a redox flow battery supporting electrolyte. For instance,the PBI gel membrane can be simply washed with water several times toremove any PA and PPA remaining in the gel membrane. For example, thePBI gel membrane can be soaked in a series of water baths, each bathretaining the PBI gel membrane for a period of time from a few minutes(e.g., about 5 minutes) to several hours (e.g., about 24 hours).Optionally, the baths can be heated, for instance, to a temperature offrom about 20° C. to about 150° C., for instance, from about 25° C. toabout 90° C., though in other embodiments, the membrane can be rinsed atambient temperature, with no particular temperature control. To confirmremoval of the PA and PPA, the pH of the wash solution can bedetermined, and washing/rinsing can continue until the pH of the washsolution is neutral.

Optionally, the PBI gel membrane can be crosslinked, which can decreasethe permeability of the membrane to redox pair ions of the batteryelectrolyte solutions without strongly affecting the desirableelectrochemical characteristics of the membranes. The manner ofcrosslinking, as well as the point in the formation process at which themembrane is crosslinked, is not particularly limited. For instance, thegel membrane can be crosslinked following rinsing/washing of theas-formed gel membrane and prior to imbibing of the membrane with asupporting electrolyte. In other embodiments, however, the membrane canbe crosslinked prior to rinsing/washing or following imbibing of themembrane with the supporting electrolyte.

In one embodiment, the PBI gel membrane can be crosslinked simply byheating in the presence of atmospheric oxygen. Crosslinking can also beaffected by the action of radiation, e.g., infrared (IR) radiation(having a wavelength of from about 700 nm to about 1 mm) including nearIR (radiation having a wavelength of from about 700 to about 2000 nm oran energy in the range from about 0.6 to about 1.75 eV).

To effect crosslinking, the PBI polymer can incorporate reactivefunctionality on the polymer chains so as to crosslink with itself oralternatively in conjunction with a crosslinking agent, i.e., apolyfunctional compound that can react with one or more functionalitiesof the PBI polymer (e.g., amines). Crosslinking agents can include anysuitable functionality to effect crosslinking. Suitable crosslinkingagents are not particularly limited, examples of which can include,without limitation, epichlorohydrin, diepoxides, diisocyanates,α,ω-dihaloalkanes, diacrylates, and bisacrylam ides, particular examplesof which can include, without limitation, α,α′-dichloro-p-xylene,chloromethyl methyl ether, bis(chloromethyl) ether, terephthaloylchloride, succinyl chloride, and dimethyl succinate, as well ascombinations of crosslinking agents. In one embodiment, from 1 to 20equivalents of crosslinking agent can be utilized per available aromaticring, but crosslinked embodiments of the membranes are not limited toany particular crosslink density.

To form the redox flow battery membrane, the gel membrane can be imbibedwith a supporting electrolyte. The supporting electrolyte of choice cangenerally depend upon the particular characteristics of the redox flowbattery in which the membrane is to be employed, and can include acidicsupporting electrolytes, basic supporting electrolytes, as well asneutral species (e.g., water). For instance, the membrane can be imbibedwith a mineral acid (e.g., a strong inorganic acid) such as hydrochloricacid, nitric acid, fluorosulfonic acid, or sulfuric acid, or a mixturethereof, or a strong organic acid such as acetic acid, formic acid,p-toluene sulfonic acid, or trifluoromethane sulfonic acid or mixturesthereof, as well as mixtures of different types of acids, e.g., acombination of a mineral acid and an organic acid. Other examples ofsupporting electrolytes that can be imbibed in the membrane can include,without limitation, sodium chloride, potassium chloride, sodiumhydroxide, potassium hydroxide, sodium sulfide, potassium sulfide, andcombinations thereof. By way of example, a supporting electrolyte caninclude H₂SO₄, HBr, HBr/HCl mixtures, HCl, NaS₂, NaS₂/NaBr mixtures, Br₂in HBr, Br₂ in H₂SO₄, Br₂ in HBr/H₂SO₄ mixtures, etc. Tetraalkylammoniumsupporting cations can be imbibed in the membranes in one embodiment,with Et₄N⁺ and Bu₄N⁺ being two non-limiting examples. A solution of atetrafluoroborate (BF⁴⁻, perchlorate (ClO⁴⁻), or hexafluorophosphate(PF⁶⁻), or a combination thereof are additional examples of supportingelectrolytes that can be imbibed in the membranes.

The concentration of the supporting electrolyte in the membrane is notparticularly limited, and in general, a solution that is imbibed in themembranes can include the supporting electrolyte in a concentration ofup to about 25 moles/liter (M), for instance, from about 0.1 M to about25 M, from about 0.5 M to about 10 M, or from about 1 M to about 5 M insome embodiments.

The membrane can be imbibed with the supporting electrolyte according toany suitable methodology. For example, the membrane can be imbibed withthe supporting electrolyte in one embodiment by soaking the membrane ina solution of the supporting electrolyte for a period of time from a fewminutes up to hours or days, optionally in an environment of increasedtemperature.

A redox flow battery membrane can include one or more additives that canbe incorporated in the membrane at the time of membrane formation or inconjunction with the supporting electrolyte. By way of example, anorganic small molecule, such as small C1-C4 alcohols (e.g., glycerol),small organic acids, urea, etc. can be incorporated in the redox flowbattery in conjunction with the supporting electrolyte.

In one embodiment, a redox flow battery membrane can incorporate aparticulate, e.g., a titanium dioxide or a PBI particulate, generally inan amount of about 2 wt. % or less, which can decrease the porosity ofthe membranes. For instance, nano-sized particulates of PBI can beincorporated into the polymeric matrix during solidification of thegel-membrane by addition of the particulate to the polymer solutionprior to or during hydrolysis.

A redox flow battery membrane including a PBI gel membrane and asupporting electrolyte can be incorporated in a redox flow battery forany use and in conjunction with any suitable electrolyte solutions andredox pairs. For instance, redox flow battery membranes as described canbe incorporated in batteries for use in the renewable energy sectorand/or in current power grids for backup/reducing energy interruptionduring peak usage times.

One embodiment of a redox flow battery cell 10 that can incorporate aredox flow battery membrane 12 as described herein is illustrated inFIG. 1. As shown, the cell can be in liquid communication with a firsttank 100 that can retain a first electrolyte solution and a second tank200 that can retain a second electrolyte solution. The tanks 100, 200can be in liquid communication with either side of a redox flow batterymembrane 12 of the cell 10 by use of conduits 110, 210, pumps 112, 212,valves, control systems, etc. The electrolyte solutions stored in thetanks 100, 200 can be circulated into either side of the cell 10 tocontact either side of the membrane 12 by pumps 112 and 212,respectively, during charging and discharging.

The electrolyte solutions of a battery can each incorporate one memberof a redox pair, as is known. In one particular embodiment, a redox flowbattery membrane can be utilized in VRB, as is known in the art. A VRBincludes in a first electrolyte solution a vanadium-based compound inwhich the vanadium alternates between a +5-valent (pentavalent) and a+4-valent (tetravalent) vanadium such as, for example, (VO₂)₂SO₄,VO(SO₄), or a combination thereof. The second electrolyte solution caninclude as active material vanadium-based compound in which the vanadiumalternates between a +2-valent (divalent) to +3-valent (trivalent)vanadium, such as, for example, VSO₄, V₂(SO₄)₃, or a combinationthereof.

The charge/discharge chemical reactions a VRB can be represented in oneembodiment as:

Positive Electrode:

VO²⁺+H₂O−e ⁻→VO₂ ⁺+2H⁺(charge)

VO²⁺+H₂O−e ⁻←VO₂ ⁺+2H⁺(discharge)

-   -   E⁰=+1.00 V vs. standard hydrogen electrode (SHE)

Negative Electrode

V³⁺ +e ⁻→V²⁺ (charge)

V³⁺ +e ⁻←V²⁺ (discharge)

-   -   E⁰=−0.26 V vs. SHE

Overall Chemical Reaction:

VO²⁺+V³⁺+H₂O→VO₂ ⁺+2H⁺+V²⁺ (charge)

VO²⁺+V³⁺+H₂O←VO₂ ⁺+2H⁺+V²⁺ (discharge)

-   -   E⁰ _(cell)=1.26 V vs. SHE

Of course, the redox flow batteries described herein are not limited toVRB, and other batteries including other redox pairs are encompassedherein. Exemplary redox pairs can include, without limitation, Zn/Br₂;Zn/Fe; Fe/Cr; polysulfide/Br₂; polysulfide/12;9,10-anthraquinone-2,7-disulphonic acid (AQDS)/Br₂; Poly(methylviologen) (poly(MV))/poly(2,2,6,6-tetramethylpiperidinyloxy-4-ylmethacrylate) (poly(TEMPO)); bis-(trimethylammonio)propyl vionlogentetrachloride (BTMAP-Vi)/BTMAP-ferrocene dichloride (BTMAP-Fc);2,6-dihydroxyanthraquinone (2,6-DHAQ)/ferrocyanide; andalloxazine7/8-carboxylic acid (ACA)/ferrocyanide.

By way of example, in one embodiment, a battery can include anelectrolyte system that includes as an active anolyte material aferrocyanide such as [Fe(CN)₆]₃/[Fe(CN)₆]₄ and as an active catholytematerial Fe²⁺ and Fe³⁺. The catholyte in such a system can include aniron/ligand complex, examples of which can include, without limitation,triethanolamine, diethanolamine, ethanolamine,N,N-bis-(2-hydroxyethyl)-(iminotris)-(hydroxymethyl)-methane, andmixtures thereof in which the catholyte may have a ligand-to-iron ratioof from about 3:1 to about 10:1.

The electrolyte solutions can generally include the active material(e.g., vanadium ion, iron ion, etc.) in a concentration of from about0.5 M to about 10 M. For instance, an electrolyte solution can includean active material in a concentration of at about 0.5 M or more, about0.6 M or more, or about 0.7 M or more, for instance, from about 1 M toabout 3 M.

In one embodiment, the electrolyte solutions can include the respectiveactive material at a concentration of the active material in a rangefrom 1 M to 10 M. In one embodiment, when the active material has aconcentration within this range, it can encourage the high-energydensity and high-power density under which the redox flow batterymembranes are capable of operating. In one embodiment, when the activematerial has a concentration of less than 1 M, the active materialincluded in the liquid can be too little of an amount per unit volume,thereby decreasing energy density. In one embodiment, when the activematerial has a concentration of more than 10 M, the electrolyte solutioncan have a sharply increased viscosity, and thus, a remarkably decreasedoxidation/reduction reaction speed, thereby decreasing power density.The paired electrolyte solutions of a redox flow battery can includetheir respective redox pair active materials in the same concentrationas one another or in different concentrations, with the preferredconcentrations generally depending upon the particular redox pair to beutilized, the application of the battery, and the presence of anyadditional additives in the electrolyte solutions.

The electrolyte solutions of a battery can include additives, such asone or more redox flow battery supporting electrolytes as discussedpreviously. In one embodiment, the electrolyte solutions of a batterycan include the supporting electrolyte that has been imbibed in theredox flow battery membrane.

An electrolyte solution can include a sulfuric acid supportingelectrolyte in one embodiment. For instance, an electrolyte solution caninclude a mixture of sulfuric acid and water, that is, a sulfuric acidaqueous solution, in conjunction with the active material of thesolution, for instance, as a solvent. In one embodiment, a mixture of asupporting electrolyte and water, e.g., a sulfuric acid aqueoussolution, can include a supporting electrolyte in a concentration offrom about 1 M to about 5 M. The concentration of the supportingelectrolyte can be selected in one embodiment so as to provide suitablesolubility for the active material of the electrolyte solution. As such,the solution can exhibit desirable ion conductivity and viscosity andcan avoid creating an overvoltage issue in the battery.

As indicated in FIG. 1, each side of a cell 10 can include additionalcomponents adjacent the membrane 12 as are known in the art including aconductive separator 14, e.g., a porous carbon paper, carbon cloth,carbon felt, or metal cloth (a porous film made of fiber-type metal or ametal film formed on the surface of a polymer fiber cloth), amongothers. The cell can also include electrodes 16, as are known, which maybe the same or different from one another and may be made of aconductive substrate appropriate for the respective electrolyte solutionof the cell (e.g., graphite). Current collectors 18 (e.g., gold-platedcopper) can be in electrical communication with the electrodes 16, and acell can include end plates 20 (e.g., stainless steel end plates), oneon either side of the ½ cell, and facing oppositely away from aseparator. The current collectors 18 provide electrical communicationbetween the cell 10 and an exterior circuit, as shown.

FIG. 2 illustrates a plurality of cells 10 arranged in a typical cellstack 150 of a redox flow battery. As shown, a first circulation path300 can pass through one side of each of the cells 10 of the stack 150such that the electrolyte solution of this half of the battery flowsthrough the path 300 and returns to the first tank 100. A secondcirculation path 400 passes through the other side of each of the cells10 of the stack 150 such that the electrolyte solution of this half ofthe battery flows through the path 400 and returns to the second tank200. As indicated in FIG. 1, a redox flow battery can further includerespective charging/discharging circuits, as well as converters,controllers, etc., as are known in the art to collect and supply powerby use of the battery.

Redox flow battery membranes as described herein can allow for higherperforming flow batteries operating under high current loads. Suchimproved operating conditions can mitigate the need for largeelectrochemical stacks, and thereby can reduce the overall cost ofcommercial flow battery devices. Further, the membranes of the presentinvention exhibit superior performance due to high ionic conductivity.This, in-turn, can reduce the cost of the overall build by reducing thenecessary stack size.

The present disclosure may be better understood with reference to theExamples set forth below.

Materials and Methods

3,3′,4,4′-Tetraaminobiphenyl (TAB, polymer grade, —97.5%) was donated byBASF® Fuel Cell, Inc. and used as received. Additional monomers werepurchased and used as received. PPA (115%) was supplied from FMCCorporation and used as received. α,α′-Dichloro-p-xylene (>98.0% purity)was purchased from TCI and used as received.

Polymer Synthesis and Membrane Fabrication

A typical polymerization included combination of the monomers and thePPA solvent, mixing with an overhead stirrer and purging with drynitrogen. The contents of the reaction kettle were heated in a hightemperature silicone oil bath, and the temperature was controlled by aprogrammable temperature controller with ramp and soak features. Oncethe reaction was complete, determined by visual inspection of viscosity,the polymer solution was cast onto clear glass plates using a doctorblade with a controlled gate thickness of 15 mils unless otherwisenoted. The cast solution was hydrolyzed into membranes in a humiditychamber regulated to 55% relative humidity at 25° C.

Acid Exchange

As-cast membranes were placed in DI water baths, and the pH of the waterwas monitored using pH strips. Water baths were replaced every 8 hoursuntil a pH of 7 was recorded. At this point, the membrane was eitherplaced into a 2.6 M sulfuric acid bath for 24 hours to ensureequilibrium of acid doping, or the membrane was further modified by acrosslinking reaction.

Post-Membrane Formation Crosslinking

After PA removal from the PBI gel membranes they were allowed to soak ina bath of 0.0523 M solution of α,α′-dichloro-p-xylene in methanol. Thebath was covered, heated to 30° C., and agitated with a magnetic stirbar. Crosslinking reactions were typically allowed to proceed for 6hours. The membrane was then washed with DI water and methanolcyclically, at least three times. The membrane was then transferred to a2.6 M sulfuric acid (SA) bath for 24 hours for acid doping.

Membrane Composition

The composition of sulfuric acid-doped PBI membranes was determined bymeasuring the relative amounts of polymer solids, water, and acid in themembranes. The sulfuric acid content of a membrane was determined bytitrating a membrane sample with standardized sodium hydroxide solution(0.10 M) using a Metrohm™ 888 DMS Titrando® autotitrator. Once titrated,the sample was thoroughly washed with DI water and dried at reducedpressures at 120° C. overnight. The dried sample was then weighed todetermine the polymer solids content of the membrane.

Using Equations 1 and 2, the polymer weight percentage and sulfuric acidweight percentage were determined, respectively;

$\begin{matrix}{{{Polymer}w/w\%} = {\frac{W_{dry}}{W_{sample}} \cdot 100}} & (1)\end{matrix}$ $\begin{matrix}{{{Acid}w/w\%} = \frac{M_{acid} \cdot V_{NaOH} \cdot c_{NaOH}}{2 \cdot W_{sample}}} & (2)\end{matrix}$

where W_(sample) is the weight of the sample before titration, W_(dry)is the weight of final dried sample after titration, M_(acid) is themolecular weight of sulfuric acid, and V_(NaOH) and C_(NaOH) are thevolume and concentration of the sodium hydroxide solution required toneutralize the sulfuric acid to the first equivalence point. It isimportant to note that even though the second proton of sulfuric acid ismuch less acidic than the first, it is still a strong enough acid tocause both protons to be titrated simultaneously, pK_(a1)=−3 andpK_(a2)=2.

The number of moles of sulfuric acid per mole of PBI repeat unit (or theSA doping levels, X) were calculated from the equation:

$\begin{matrix}{X = \frac{V_{NaOH} \cdot c_{NaOH}}{{2 \cdot W_{dry}}/M_{polymer}}} & (3)\end{matrix}$

where V_(NaOH) and C_(NaOH) are the volume and concentration of thesodium hydroxide solution required to neutralize the sulfuric acid tothe first equivalence point, W_(dry) is the final weight of the driedsample after titration, and M_(polymer) is the molecular weight of thepolymer repeat unit.

Conductivity

In-plane conductivity of the membrane was measured by a four-probeelectrochemical impedance spectroscopy (EIS) method using a FuelCon(TrueData®-EIS PCM) electrochemical workstation over the frequency rangefrom 1 Hz to 50 kHz. A membrane sample with a typical geometry of 1.0cm×4.0 cm was fixed into the measuring 4-electrode head of themeasurement. The conductivity of the membrane was calculated using thefollowing equation:

$\begin{matrix}{\sigma = \frac{d}{l \cdot w \cdot R_{m}}} & (4)\end{matrix}$

where d is the distance between the two inner probes, l is the thicknessof the membrane, w is the width of the membrane, and R_(m) is the ohmicresistance determined by the model fitting. Conductivities wereconducted at room temperature, to replicate normal operating conditionsof VRBs.

Vanadium Permeability

The crossover of vanadium(IV) was measured utilizing a PermeGear®“side-by-side” direct permeation cell. The cell had two chambers with a45 mL volume separated by the membrane under test. The temperature ofthe chambers was regulated at 25° C. with a recirculating water bath. Atypical test experiment contained 1.5 M VOSO₄ in 2.6 M sulfuric acid inthe donor chamber, and 1.5 M MgSO₄ in 2.6 M sulfuric acid in thereceptor chamber. Vanadium(IV) has a strong absorption characteristic at248 nm. Utilizing this property, the concentration of the receptorchamber was measured with a Shimadzu® UV-2450 UV/Vis spectrometer atvarious time intervals. The VO²⁺ permeability was calculated usingFick's diffusion law, Equation 5:

$\begin{matrix}{{P_{s}t} = {{\ln\left\lbrack {1 - {2\frac{c_{r}(t)}{c_{r}(0)}}} \right\rbrack}\left\lbrack {- \frac{Vd}{A}} \right\rbrack}} & (5)\end{matrix}$

where c_(r)(t) is the receptor VOSO₄ concentration at time t, c_(r)(0)is the donor initial VOSO₄ concentration, V is the donor and receptorsolution volume, d is the membrane thickness, A is the active area ofthe membrane, and P_(s) is the salt permeability.

Vanadium Test Cell

A VRB test cell was assembled with an active area of 24 cm² and utilizedinterdigitated flow fields for liquid electrolyte solutions machinedinto carbon plates. During use, a membrane was sandwiched betweenidentical commercial carbon paper electrodes that had been previouslyheat treated to 400° C. for 30 hours in air, and gasketed withpolytetrafluoroethylene (PTFE) films. The cell was equipped with tworeservoirs of 100 mL electrolyte solution per side consisting of 1.60 Mvanadium species having 3.55 average oxidation state and 4.2 M totalsulfur content. The electrolytes were circulated though the cell at aconstant flow rate of 120 mL/min by two acid-resistant diaphragm pumps.The charge/discharge cycling performance was measured at constantcurrent densities ranging from 72 mA/cm² to 484 mA/cm² using amulti-channel potentiostat (Model BT2000, Arbin Instruments Inc.,College Station, Tex.).

Example 1

10.71 g TAB (50 mmol), and 13.44 g monosodium 2-sulfoterephthalate(s-TPA, 50 mmol) were added to 580 g PPA and polymerized as describedabove according to the following reaction scheme to form s-PBI:

The polymerization was conducted for 48 hours in a nitrogen atmosphereat 220° C. The solution was applied by means of a doctor blade with a15-mil gate thickness to a glass plate and subsequently hydrolyzed formore than 24 hours. The membrane was rinsed several times in deionizedwater baths to remove PA hydrolysis product. pH indication paper wasused to ensure all the acid had been removed before proceeding.

A non-crosslinked membrane was formed in which a neutralized membranewas placed in a solution of 2.6 M sulfuric acid for 24 hours beforecharacterization.

A crosslinked membrane was formed in which a neutralized membrane wassubmerged in a 0.0523 M solution of α,α′-dichloro-p-xylene in methanoland stirred for 6 hours at 30° C. The membrane was washed with deionizedwater followed by methanol four times, then once more in water. Thecrosslinked membrane was placed in a solution of 2.6 M sulfuric acid for24 hours before characterization. The reaction scheme for thecrosslinking reaction was as follows:

The room temperature ionic conductivity of the membranes was evaluatedin both 2.6 M sulfuric acid and a V(IV)/H⁺ solution found in typicaloperating cell conditions. The ex-situ membrane properties for the s-PBIgel membranes (both uncrosslinked and crosslinked) are shown in Table 1.

TABLE 1 VO²⁺ Permeability Conductivity Conductivity % Polymer % SulfuricMembrane (cm² · s⁻¹) (mS · cm⁻¹)^(a) (mS · cm⁻¹)^(b) Solids^(a) Acid^(a)% Water^(a) s-PBI 5.74 × 10⁻⁷ 593 242 18.8 23.11 58.1 s-PBI-x 5.23 ×10⁻⁷ 537 240 30.6 35.6 33.8 ^(a)After soaking in 2.6M sulfuric acid^(b)After soaking in V(IV)/H⁺ solution (1.5M VOSO₄ + 2.6M sulfuric acid)for 3 days

A VRB test cell was assembled as described above including thenon-crosslinked s-PBI membrane. The VE, CE. and EE were measured withresults shown in FIG. 3 and Table 2, below. The polarization curve with80% state-of-charge electrolyte and compared to m-PBI described inComparative Example 1 is shown in FIG. 4.

TABLE 2 Current Density (mA/cm²) C.E. (%) E.E. (%) V.E. (%) 72 83.4779.58 95.34 242 93.41 81.00 86.71 483 94.45 73.06 77.35

Another VRB test cell was assembled as described above including thecrosslinked s-PBI membrane. The VE, CE. and EE were measured, and theresults are shown in FIG. 5 and Table 3, below. The polarization curvewith 80% state-of-charge electrolyte and compared to m-PBI described inComparative Example 1 is shown in FIG. 6.

TABLE 3 Current Density (mA/cm²) C.E. (%) E.E. (%) V.E. (%) 72 88.6984.11 94.80 242 96.30 81.42 84.49 483 97.00 70.51 72.55

The s-PBI gel membranes exhibit surprisingly high conductivities ascompared to the m-PBI membranes in both sulfuric acid and the acidelectrolyte solution (see Comparative Example 1); 537-593 mS·cm⁻¹compared to 13.1 mS·cm⁻¹ and 240-242 mS·cm⁻¹ compared to 12.2 mS·cm⁻¹,respectively. The slight difference in conductivity between the twos-PBI membranes is likely a result of crosslinking. The crosslinkerforms bonds with the imidazole nitrogen and may slightly inhibit aprotons path through the hydrogen bond networks.

To confirm crosslinking occurred, a 50 mg samples of neutralized driedmembranes were heated in 800 mL N,N′-dimethylacetamide at reflux for 48hours. Under these conditions, no membrane deterioration or solutioncolor change was observed for the crosslinked sample, but dissolutionwas observed for the pristine polymer film. Furthermore, the swellratios of the crosslinked vs. non-crosslinked membranes affordednoteworthy results. Utilizing a non-acid solvent(N,N′-dimethylacetamide), to ensure that unwanted solvent polymerinteractions were suppressed, it was found that the non-crosslinkedPPA-formed membrane (3.94 wt. % increase) absorbed approximately 0.75wt. % more solvent than the crosslinked membrane (3.25 wt. % increase).Restriction of chain mobility by chemical crosslinks inhibits solventswelling of the polymer gel, resulting in lower weight increase fromsolvent uptake.

The oxidative stability of the membranes was also examined, and themembranes showed no degradation in the oxidative vanadium solutions.

When comparing results for the two different imbibed solutions (sulfuricacid and a V(IV)/H⁺ solution), the decrease in conductivity of the gelmembranes in the vanadium electrolyte solution was believed to occurfrom two factors. The first being that vanadium ions may interact withthe membrane by attractive forces with the negatively charged sulfonategroup (pKa˜−2), impeding the flow of protons. The drop in conductivityin the PBI gel membranes for this solution was also attributed to theintrinsic conductivity of the electrolyte solution containing vanadiumions. Since the major contributor of proton conductance is the mobilityof ions, it was not surprising that an increase in vanadiumconcentration would diminish proton conductivity of the electrolytesolution solely with regards to an increase in viscosity of theelectrolyte solution. PBI gel membranes are believed to have aconsiderably open morphology that enhances proton conductivity byallowing not only proton transport via the Grotthuss mechanism but alsomobility of the electrolyte in the membrane; thus, proton transportthrough the membrane could be affected by the increase in viscosity dueto the incorporation of vanadium ions.

The electrolyte mobility in the PBI gel membrane is also a plausibleexplanation for the determined vanadium permeability. The permeabilitiesare not unexpected when considering the polymer solids of the membrane.The gel membrane included a relatively small amount of polymer per theamount of electrolyte in the membrane as compared to the traditional PBImembrane. Chemically crosslinking of the PBI chains in the gel membraneswas expected to fill interstitial space and limit chain mobility. Atfirst glance, the permeability of the crosslinked PBI gel membrane isless than ideal; however, this modification did have an impact whencompared to the un-modified version and without having a dramatic effecton conductivity. Since this technique is impartial to the PBI derivativeof choice, it could be used to hone the properties of PBI membranes asneeded.

Example 2

para-PBI membrane made as described above by polymerizing TAB withterephthalic acid (TPA) according to the PPA process was rinsed severaltimes in deionized water baths to remove PA. pH indication paper wasused to ensure all the acid had been removed before proceeding. Theneutralized membrane was then placed in a solution of 2.6 M sulfuricacid for at least 24 hours before characterization. In-plane ionicconductivity was measured at room temperature to be 398 mS/cm.

Example 3

A crosslinked para-PBI membrane was prepared from a membrane formed asdescribed in Example 2. 5.5016 g of α,α′-dichloro-p-xylene (31.43 mmol)was weighed and dissolved in 600 mL of methanol to form a solution thatwas poured into a glass dish. The neutralized membrane was washed threetimes with methanol before adding it to the solution in the glass dish.The mixture was covered, heated to 30° C. and stirred with a magneticstir bar overnight. Then, the crosslinked membrane was removed andwashed with methanol using a rinse bottle. It was washed several timeswith deionized water, followed by another wash of methanol and deionizedwater. The crosslinked membrane was placed in a solution of 50 wt. %sulfuric acid for 24 hours and then hot pressed for 5 minutes at 140°C., which reduced the thickness from 0.34 mm to 0.09 mm. The hot-pressedcrosslinked membrane was placed in a solution of 2.6 M sulfuric acid for24 hours before characterization. In-plane ionic conductivity wasmeasured at room temperature to be 492 mS/cm.

The crosslinked para-PBI membrane was tested in the VRB test cell asdescribed. VE, CE and EE were measured and recorded as illustrated inFIG. 7 and shown in Table 4, below.

TABLE 4 Current Density (mA/cm²) C.E. (%) E.E. (%) V.E. (%) 72 88.6984.11 94.80 242 96.30 81.42 84.49 483 97.00 70.51 72.55

Example 4

A para-PBI membrane made by the PPA process as described above was firstrinsed several times in deionized water baths to remove PA. pHindication paper was used to ensure all the acid had been removed beforeproceeding. In a 1000 mL Erlenmeyer flask, 7.88 g of4,4′-bis(chloromethyl)biphenyl (31.38 mmol) was dissolved in 600 mL ofN,N-dimethylacetamide (DMAc). The solution was poured into a glass dish,to which the membrane was added. This cross-linking reaction was stirredfor 6 hours at 80° C. The membrane was subsequently washed withdeionized water followed by methanol four times, then once more withwater. The crosslinked membrane was placed in a solution of 2.6 Msulfuric acid for 24 hours before characterization. In-plane ionicconductivity was measured at room temperature to be 367 mS/cm.

Example 5

A para-PBI membrane made by the PPA process as described above was firstrinsed several times in deionized water baths to remove PA. pHindication paper was used to ensure all the acid had been removed beforeproceeding. Then, 7.5 g of (3-glycidoxypropyl) trimethoxysilane (31.73mmol) was dissolved in 600 mL of methanol to make a 0.0523 M solution,which was then poured into a glass dish containing the neutralizedmembrane. The crosslinking reaction was performed for 2 hours at roomtemperature. The membrane was then placed directly into a solution of2.6 M sulfuric acid for at least 24 hours before characterization.In-plane ionic conductivity was measured at room temperature to be 537mS/cm.

Example 6

42.854 g of 3,3′,4,4′-tetraaminobiphenyl (TAB, 200 mmol), 4.153 g of TPA(25 mmol), 29.073 g of isophthalic acid (IPA, 175 mmol), and 685 g ofPPA were added to 3-neck 1000 mL resin kettle (7:1 molar ratio ofIPA:TPA, 10 wt. % monomer charge) equipped with an overhead mechanicalstirrer. The polymerization was conducted for 24 hours in a nitrogenatmosphere with an oil bath temperature of 220° C. The temperature wasthen decreased to 180° C., a reflux condenser was attached, and 103 g ofdeionized water was slowly added to make a 7.82 wt. % stabilized polymersolution. The solution was stirred for another 8 hours. 100.5 g of thepolymer solution and 3.6355 g of a meta-PBI powder with 100 meshparticle size were added to a resin kettle (1:0.5 weight ratio ofpolymer in solution to 100 mesh PBI powder) with a 3-neck lid andoverhead mechanical stirrer. Under a nitrogen atmosphere, the mixturewas heated to 165° C. for 4 hours, then increased to 200° C. for 30minutes before casting with a 15-mil doctor blade onto a glasssubstrate. It was allowed to hydrolyze into a membrane in a humiditychamber regulated to 55% R.H. at 25° C. for more than 24 hours. Themembrane was then rinsed several times in deionized water baths toremove PA. pH indication paper was used to ensure all the acid had beenremoved. The neutralized membrane was placed in a solution of 2.6 Msulfuric acid and for at least 24 hours before characterization.In-plane ionic conductivity was measured at room temperature to be 175mS/cm.

The VE, CE. and EE were measured in a vanadium test cell as described.Results are shown in FIG. 8 and Table 5, below.

TABLE 5 Current Density (mA/cm²) C.E. (%) E.E. (%) V.E. (%) 72 95.7089.63 93.63 242 98.09 79.58 81.07 483 98.48 65.45 66.35

Example 7

6.8811 g of TAB (32.11 mmol), 0.6673 g of TPA (4.01 mmol), 4.6689 g ofisophthalic acid (IPA, 28.10 mmol), and 110 g of PPA were added to 100mL reaction kettle (7:1 ratio of IPA:TPA, 11 wt. % monomer charge) andequipped with an overhead mechanical stirrer. The polymerization wasconducted for 24 hours in a nitrogen atmosphere with an oil bathtemperature of 220° C. The solution was applied by means of a doctorblade with a 20-mil gate thickness to a glass plate and subsequentlyhydrolyzed for more than 24 hours. The membrane was rinsed several timesin deionized water baths to remove PA. pH indication paper was used toensure all the acid had been removed before proceeding. It was thensubmerged in a 1:1 solution of isopropanol and 85 wt. % PA and stirredfor 24 hours. It was removed from the mixture and left to evaporate onthe bench top for 30 minutes. The membrane was soaked in 2.6 M sulfuricacid solution for 24 hours before characterization. In-plane ionicconductivity was measured at room temperature to be 228 mS/cm.

A VRB test cell was assembled as described above. The VE, CE and EE weremeasured. Results are shown in FIG. 9 and Table 6, below.

TABLE 6 Current Density (mA/cm²) C.E. (%) E.E. (%) V.E. (%) 72 88.4784.06 95.00 242 96.12 81.71 84.94 483 97.58 71.10 72.76

Example 8

1.778 g TAB (8.30 mmol), 2.226 g s-TPA (8.30 mmol) and 96 g of PPA wereadded to 100 mL reaction kettle (4 wt. % monomer charge) equipped withan overhead mechanical stirrer. The polymerization was conducted for 48hours in a nitrogen atmosphere at 220° C. The solution was applied bymeans of a doctor blade with a 15-mil gate thickness to a glass plateand subsequently hydrolyzed for more than 24 hours. The membrane wasrinsed several times in deionized water baths to remove PA. pHindication paper was used to ensure all the acid had been removed. 700mL of methanol and 6.543 g of α,α′-dichloro-p-xylene (37.38 mmol) weremixed to form a solution to which the neutralized membrane was added andstirred at room temperature for 24 hours. The membrane was then removedfrom the solution and washed with deionized water and methanol multipletimes. It was placed in a deionized water bath and then in a 2.6 Msulfuric acid bath for 24 hours before characterization. In-plane ionicconductivity was measured at room temperature to be 289 mS/cm.

Example 9

5.937 g TAB (27.71 mmol), 5.490 g of 2,5-dihydroxyterephthalic acid(diOH-TPA, 27.71 mmol), and 362.32 g PPA were added to a reaction kettleand stirred under a nitrogen atmosphere with an overhead mechanicalstirrer. The polymerization was conducted for 24 hours in a nitrogenatmosphere at 220° C. The solution was applied by means of a doctorblade with a 20-mil gate thickness to a glass plate and subsequentlyhydrolyzed for more than 24 hours. The membrane imbibed in PA was rinsedseveral times in deionized water baths. pH indication paper was used toensure all the acid had been removed before proceeding. The neutralizedmembrane was placed in a solution of 2.6 M sulfuric acid and stirred for24 hours before characterization. In-plane ionic conductivity wasmeasured at room temperature to be 608 mS/cm.

Example 10

5.9599 g TAB (27.81 mmol), 2.3104 g of IPA (13.91 mmol), 3.7297 g of5-sulfoisopthalic acid monosodium salt (5SIPA, 13.91 mmol) and 138 g PPAwere added to 100 mL reaction kettle (1:1 ratio of IPA:5SIPA, 8 wt. %monomer charge) equipped with an overhead mechanical stirrer. Thepolymerization was conducted for 24 hours in a nitrogen atmosphere at220° C. The solution was applied by means of a doctor blade with a20-mil gate thickness to a glass plate and subsequently hydrolyzed formore than 24 hours. The membrane was rinsed several times in deionizedwater baths to remove PA. pH indication paper was used to ensure all theacid had been removed before proceeding. The neutralized membrane wasplaced in a solution of 2.6 M sulfuric acid and stirred for 24 hoursbefore characterization. In-plane ionic conductivity was measured atroom temperature to be 109 mS/cm.

Comparative Example 1

Commercially available meta-PBI film, prepared from casting and dryingN,N-dimethylacetamide solutions, was used as received. The film wasplaced in a solution of 2.6 M sulfuric acid for 24 hours beforecharacterization. In-plane ionic conductivity in sulfuric acid wasmeasured at room temperature to be 13.1 mS/cm. In-plane ionicconductivity after soaking in a V(IV)/H⁺ solution (1.5 M VOSO₄+2.6 Msulfuric acid) for 3 days was measured at room temperature to be 12.2mS/cm. The content of polymer solids in the membrane was determined tobe 65.6%.

A VRB test cell was assembled as described above. The VE, CE and EE weremeasured and the results are shown in FIG. 10 and Table 7.

TABLE 7 Current Density (mA/cm²) C.E. (%) E.E. (%) V.E. (%) 72 99.979.28 77.96 242 — — — 483 — — —

As shown, at higher current densities, the cell had no performance andcould not be operated at current densities above about 72 mA/cm². Thisis because voltage is related to the membrane conductivity, which isvery low for this membrane.

The polarization curve with 80% state-of-charge electrolytes was alsodetermined and compared to those of Example 1, above. The comparison isshown in FIG. 11.

Comparative Example 2

Commercially available meta-PBI film, prepared from casting and dryingN,N-dimethylacetamide solutions, was used as received. A crosslinkingreaction was conducted by first mixing 1000 mL of methanol with 9.27 gα,α′-dichloro-p-xylene (52.95 mmol) to form a solution which was pouredinto a glass dish containing the membrane. The solution with themembrane was covered, heated to 30° C. and stirred with a magnetic stirbar for 24 hours. The crosslinked membrane was removed from thesolution, washed and left to dry in the open air. After drying, thecrosslinked membrane was placed in a solution of 2.6 M sulfuric acid for24 hours before characterization. In-plane ionic conductivity wasmeasured at room temperature to be 15.6 mS/cm.

Comparative Example 3

Commercially available meta-PBI film, prepared from casting and dryingN,N-dimethylacetamide solutions, was used as received. The membrane wasplaced in a bath of 85 wt. % PA for 48 hours. The membrane was rinsedseveral times in deionized water baths to remove PA. pH indication paperwas used to ensure all the acid had been removed before proceeding. Onceneutral, it was placed in a solution of 2.6 M sulfuric acid for 24 hoursbefore characterization. In-plane ionic conductivity was measured atroom temperature to be 11.6 mS/cm.

Comparative Example 4

Commercially available meta-PBI film, prepared from casting and dryingN,N-dimethylacetamide solutions, was used as received. The membrane wasplaced in a bath of 85 wt. % PA for 48 hours. A crosslinking reactionwas conducted by first mixing 1000 mL of methanol with 9.40 gα,α′-dichloro-p-xylene (53.70 mmol) to form a solution, which was pouredinto a glass dish containing the membrane. The solution with themembrane was covered, heated to 30° C. and stirred with a magnetic stirbar for 25 hours. The crosslinked membrane was then removed from thesolution and washed in methanol and deionized water before being allowedto dry in the open air. The crosslinked membrane was then placed in asolution of 2.6 M sulfuric acid for 24 hours before characterization.In-plane ionic conductivity was measured at room temperature to be 16.7mS/cm.

While certain embodiments of the disclosed subject matter have beendescribed using specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the subjectmatter.

What is claimed is:
 1. A redox flow battery membrane comprising: apolybenzimidazole gel membrane, the polybenzimidazole gel membrane beinga self-supporting membrane capable of incorporating a liquid content ofabout 60 wt. % or more without loss of structure; and a redox flowbattery supporting electrolyte imbibed within the polybenzimidazole gelmembrane; wherein the redox flow battery membrane exhibits an in-planeionic conductivity in a 2.6 M sulfuric acid solution of about 100 mS/cmor greater.
 2. The redox flow battery membrane of claim 1, wherein thepolybenzimidazole gel membrane exhibits an in-plane ionic conductivityin a 2.6 M sulfuric acid solution of about 200 mS/cm or greater.
 3. Theredox flow battery membrane of claim 1, wherein the polybenzimidazole ofthe gel membrane comprises one or more of the following repeating units:

or any combination thereof, in which n and m are each independently 1 orgreater, about 10 or greater, or about 100 or greater.
 4. The redox flowbattery membrane of claim 1, wherein the supporting electrolytecomprises a mineral acid, an organic acid, or a combination of one ormore mineral acids and/or one or more organic acid.
 5. The redox flowbattery membrane of claim 1, wherein the supporting electrolytecomprises hydrochloric acid, nitric acid, fluorosulfonic acid, aceticacid, formic acid, p-toluene sulfonic acid, trifluoromethane sulfonicacid, or any mixture thereof.
 6. The redox flow battery membrane ofclaim 1, wherein the supporting electrolyte comprises sodium chloride,potassium chloride, sodium hydroxide, potassium hydroxide, sodiumsulfide, potassium sulfide, or any combination thereof.
 7. The redoxflow battery membrane of claim 1, wherein the polybenzimidazole gelmembrane is crosslinked.
 8. The redox flow battery membrane of claim 1,wherein the polybenzimidazole gel membrane is free of organic solvents.9. The redox flow battery membrane of claim 1, wherein thepolybenzimidazole gel membrane is free of phosphoric acid andpolyphosphoric acid.
 10. The redox flow battery membrane of claim 1,wherein the supporting electrolyte comprises a tetraalkylammoniumcation.
 11. The redox flow battery membrane of claim 1, wherein thesupporting electrolyte comprises sulfuric acid.
 12. A redox flow batterycomprising a polybenzimidazole gel membrane and a redox flow batterysupporting electrolyte imbibed within the polybenzimidazole gelmembrane, the polybenzimidazole gel membrane being a self-supportingmembrane capable of incorporating a liquid content of about 60 wt. % ormore without loss of structure, wherein the redox flow battery iscapable of operation at a current density of about 100 mA/cm² orgreater.
 13. The redox flow battery of claim 12, wherein the redox flowbattery is a vanadium redox flow battery.
 14. The redox flow battery ofclaim 12, wherein the redox flow battery has a coulombic efficiency ofabout 90% or greater and/or an energy efficiency of about 75% or greaterand/or a voltage efficiency of about 80% or greater at a current densityof 242 mA/cm².
 15. The redox flow battery of claim 12, wherein the redoxflow battery has a coulombic efficiency of about 90% or greater and/oran energy efficiency of about 65% or greater and/or a voltage efficiencyof about 65% or greater at a current density of 483 mA/cm².
 16. Theredox flow battery of claim 12, wherein the polybenzimidazole gelmembrane is free of organic solvents.
 17. The redox flow battery ofclaim 12, wherein the polybenzimidazole gel membrane is free ofphosphoric acid and polyphosphoric acid.
 18. The redox flow battery ofclaim 12, wherein the supporting electrolyte comprises a mineral acid, astrong organic acid, or a combination of one or more mineral acidsand/or one or more organic acid.
 19. The redox flow battery of claim 12,wherein the supporting electrolyte comprises sodium chloride, potassiumchloride, sodium hydroxide, potassium hydroxide, sodium sulfide,potassium sulfide, or any combination thereof.
 20. The redox flowbattery of claim 12, wherein the supporting electrolyte comprisessulfuric acid.